Typically, printers require a supply of a print media, such as paper, to be loaded into the printer and controllably moved through the printer along a media path. The media is often supplied as either a continuous stream of paper or as individual sheets. The paper is usually fed into the printer using either a tractor-feed mechanism or a friction-feed mechanism. With reference to FIG. 1, the tractor-feed technique employs a continuous length of paper with holes evenly perforated along both sides. The paper is threaded into the paper transport mechanism, and inserted onto sprocket wheels 1 located just over the platen 2. The teeth on each sprocket wheel 1 mesh exactly with holes in the paper. The platen 2 and sprocket wheels 1 are mechanically linked, such that as the platen is advanced, the sprocket wheels 1 turn to pull paper through evenly. Some tractor-fed configurations can be threaded so that the paper is pushed through the printer instead of being pulled through. Pressure or contact rollers 8 are provided to hold the paper firmly against the platen 2 to ensure positive traction and even paper advance. Bail rollers 3 may also be provided to hold paper flat while the paper travels around the platen and along the paper path. The platen 2 is not only used to advance the paper but is also used to provide a support surface for the paper aligned with a print head 4. A feed guide 6 may also be provided for directing the paper 7 along the media path.
With reference to FIG. 2, a friction-feed paper transport system will work with a variety of media types, such as a continuous sheet of paper 7 wound in a roll, or a stack of individual sheets of paper (not shown). A mechanical lever (not shown) is often used to pry pressure rollers 8 away from the platen 2 while paper 10 is being threaded and aligned. When the lever is released, pressure rollers 8 are brought into tight contact with the paper 7 and the platen 2. As the platen advances, contact forces between the platen 2 and the pressure rollers 8 advance the paper along the media path. Bail rollers 3 are provided to keep paper flat and even against the platen 14 as paper leaves the printer.
The drive rollers or platen 2 often are driven by a stepper motor that drives the drive rollers in small increments or steps such that the paper 7 is propelled incrementally or stepped through the printer, pausing slightly between each step. As the paper is stepped through the printer it passes a conventional print head having a linear array of elements, such as a thermal print head or an ink jet print head. During each pause between steps, a small portion of the paper is aligned with the print head and selected elements of the print head are activated to produce a portion of an image on the portion of the paper aligned with the print head.
This image portion is a small portion of an entire image to be printed. The entire image typically is produced by stepping the paper past the print head, pausing the paper after each step, determining a step number (e.g., fifth step or sixth step) corresponding to the pause, determining the portion of the image corresponding to the step number, determining which elements of the print head to activate to produce the determined portion of the image, and activating the determined elements to produce the determined portion of the image on the paper. A microprocessor controls such operation.
Loading paper in a tractor-fed mechanism is often a difficult task. Typically, the paper must be threaded through a slot formed in the housing of the printer, and engaged with the sprocket wheels. A pair of spring-loaded guides are typically pivoted away from the sprocket wheels allowing the perforations in the paper to be placed over the sprockets of the wheels. The paper must be carefully aligned such that the sprockets engage corresponding perforations on each side of the paper. Only after the paper has been properly aligned may the guides be pivoted back into place. The guides prevent the paper from jumping off the sprockets. Access to the print head is extremely limited since the print head is often positioned in close proximity to the platen.
Similar difficulties are encountered with friction-feed mechanisms. While a friction-feed system will usually automatically thread the paper through the feed mechanism, it is often difficult to remove paper which becomes jammed in the media path. This is due to the substantially fixed distance between the print head and the platen. Access to the print head is usually limited, making repairs difficult.
Depending upon the type of print head, the print head is positioned either adjacent or proximate the media. For example, a thermal print head must be sufficiently close to the print medium to permit the heat produced by the resistive elements of the print head to warm the thermally sensitive print media or print ribbon. In another example, impact printers must actually make physical contact with the print medium or a ribbon between the print medium and the print head to form printed letters. Alternatively, an ink jet print head must be spaced sufficiently from the media to allow precise dots to be formed.